Photopolymerization of unsaturated compositions wherein a photoinitiating compound is included in the polymerizable mass is well known in the art. The process has many advantages over thermal polymerization and is particularly useful where long holding life, combined with rapid hardening at low temperature, is desirable. Photoinitiating compounds must absorb light and utilize the energy so acquired to initiate polymerization.
A large number of compounds have been found useful as photoinitiators for the polymerization of unsaturated compounds. Among those heretofore in most common usage in industry are the benzoin ethers of primary and secondary alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol and isobutyl alcohol.
While particular industrial applications often dictate certain requisite characteristics, the primary determinants of universal application in the selection of a suitable photoinitiating compound are its level of reactivity and its effect upon storage stability when combined with the photopolymerizable medium wherein it is to function. This latter characteristic is significant in view of the desirability of one-component systems which will not gel prior to use.
While compounds in common use as photoinitiators do effect rates of polymerization which are industrially acceptable and render photopolymerization superior to thermal polymerization in various applications, methods of achieving increased polymerization rates with increased stability are constantly being sought. Improved photoinitiators are particularly desirable since photopolymerization techniques are gaining increasingly widespread acceptance due to the inherently lower equipment costs, reduction of volatile emissions and reduced energy consumption which attend their use.
Thus, the ethers of benzoin, which are widely used as photoinitiating compounds, are not wholly satisfactory with regard to the one-component system storage stability factor. Any unsaturated system to which a benzoin ether is added has considerably diminished dark storage stability and will gel prematurely. Various attempts have been made to remedy this deficiency of the benzoin compounds by including stabilizing additives in the polymerization system. Thus, U.S. Pat. No. 3,819,495 discloses the addition of organic chlorine containing compounds and copper compounds as a stabilization system, while U.S. Pat. No. 3,819,496 teaches the use of organic chlorine compounds with iron and/or manganese compounds for that purpose. Many other stabilizers have been suggested and, while some improvements have been achieved in the stability of unsaturated systems containing benzoin-type photoinitiators, the necessity of incorporating stabilizing additives raises the cost of such systems appreciably, while the results are still not wholly satisfactory.
Thus, various aromatic compounds have been proposed as photoinitiators for unsaturated compounds. For example, U.S. Pat. No. 3,715,293 teaches the use of acetophenone compounds such as 2,2-diethoxyacetophenone, while a series of patents including U.S. Pat. Nos. 3,926,638; 3,926,639; 3,926,640; 3,926,641; 4,022,674; 4,004,998; 4,008,138; and 4,028,204 describe complex compounds derived from benzophenone. As an example of the benzophenone-derived materials, U.S. Pat. No. 4,004,998 describes photoinitiators made by reacting carboxy-substituted benzophenones with hydroxyl-containing polyethylenically unsaturated esters, while U.S. Pat. Nos. 3,926,639 and 4,028,204 describe a benzophenone substituted with a carboxy group and an ester group which is reacted with certain resins, such as alkyds, polyesters, polyethers, polyamides and epoxies, to provide the photoinitiator.
U.S. Pat. No. 4,180,599 discloses certain unsaturated esters prepared by esterifying 3,4,3',4'-benzophenone tetracarboxylic acid with beta-hydroxyethyl acrylate esters. These esters contain four acrylate moieties per mol and are characterized as a dual-purpose photoinitiator and crosslinking agent for UV-curable coatings.
Another approach is disclosed in U.S. Pat. No. 3,759,807 where certain benzophenones which must be used with activators are disclosed. Also representative of benzophenone systems is Brit. Pat. No. 1,223,463 which teaches the addition of diketones, such as m-benzoylbenzophenone, or ethylene glycol bis(p-benzoylbenzoate), to nylon to give photosensitive materials suitable for the preparation of printing plates.
In U.S. Pat. No. 4,017,652, ethyl benzoylbenzoate is disclosed as a photosensitizer which must be used in connection with a photoinitiator such as a benzoin ether.
With regard to rate of polymerization and the dark storage stability of the uncured system, none of the most widely used photoinitiating compounds is wholly acceptable in unsaturated systems.